The structure of cytochrome b sub minus 562 from E. coli will be studied by x-ray diffraction methods. Both the oxidized and reduced protein will be examined. The major goal will be to understand the mechanism of electron transport at the molecular level and to apply this understanding to other systems, such as the insoluble b sub minus cytochromes of mitochondria. A second objective is to compare its structure to other heme proteins of known structure such as cytochrome b sub minus 5, cytochrome c sub minus and myoglobin. Ferricytochrome b sub minus 562 crystallizes in a triclinic unit cell with two molecules per asymmetric unit. The crystals are suitable for x-ray analysis and the data can be measured to 2.0 A resolution. Crystals of ferrocytochrome b sub minus 562 have also been prepared which are not yet large enough to characterize by x-ray diffraction. The location of the two independent iron atoms in the unit cell have been obtained from the anomalous dispersion difference Patterson. The relative orientation of the two molecules has been determined from the Rossmann rotation function. One heavy atom derivative has been solved and a second has been found. X-ray phases based on the isomorphous derivatives and the molecular replacement method will be used to calculate an electron density map first at 4.5A resolution and later at 2.8A resolution.